1. Field of the Invention
The present invention generally relates to compound semiconductor integrated circuit devices, and more particularly, to a hetero-junction bipolar transistor.
2. Description of the Related Art
A conventional hetero-junction bipolar transistor (hereinafter referred to as HBT simply) having an AlGaAs emitter layer is known. An emitter contact layer is provided on the AlGaAs emitter layer to make an ohmic contact with the emitter layer. An emitter electrode containing gold (Au) is provided on the emitter contact layer. A barrier layer made of a refractory metal such as titanium (Ti) may underlie the emitter electrode in order to prevent Au of the emitter electrode from diffusing into the AlGaAs emitter layer.
However, the barrier metal made of the refractory metal cannot prevent diffusion of Au into the AlGaAs emitter layer perfectly. Au diffused into the emitter layer may induce electromigration, which degrades the performance of the transistor. Further, Au of the emitter electrode may diffuse into a base layer that underlies the emitter layer. Au diffused into the base layer may result in a base leakage current. Thus, the conventional HBT is not reliable for a long period of time.
It is a general object of the present invention to provide a hetero-junction semiconductor bipolar transistor in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a hetero-junction semiconductor bipolar transistor in which diffusion of Au into an epitaxial layer can be prevented and high reliability can be secured for a long term.
The above objects of the present invention are achieved by a hetero-junction bipolar transistor comprising: a collector layer, a base layer and an emitter layer, an emitter electrode containing Au being provided for the emitter layer; and an Au-diffusion barrier layer of one of InP and InGaP interposed between the emitter electrode and the base layer.